The utility of synthetic polymers as replacement material for various types of human tissue has been substantially advanced by recent developments in improved compatability characteristics of polymer compositions with the numerous chemical environments of the body. Although improvements in body tissue and blood compatibility (hereinafter referred to as "compatibility") have enabled more extensive use of synthetic polymers in prosthetic surgery, the continued failure of many of these implants has greatly impeded progress in treatment of persons requiring replacement of human tissue.
It has now been discovered that many of these failures were the product of mechanical mis-match, as well as compatability rejection. Inasmuch as the effects of these two causes are quite similar--clotting of the blood and tissue rejection--the subsequent failure of the newer nonthrombogenic materials was viewed as simply a further rejection due to other chemically adverse reactions to the prosthetic implant.
Recent investigation of vascular grafts has disclosed, however, that such thrombosis and accompanying graft failure of these nonthrombogenic materials was the result of mechanical mis-match between the graft and natural tissues. Such mis-match includes a variance in elastic response and other physical properties such that the grafted material does not mechanically respond in consonance with the natural tissue. The resulting traumatization and other adverse tissue reactions cause clotting and occlusion of the vein, similar to that experienced with chemical noncompatibility.
These effects of compliance mis-match are particularly troublesome in small diameter vascular grafts. The continual variations of blood pressure cause a recurring pulsing motion resulting in constant expansion and contraction of the vascular tissues. Where the grafted material is not of an equivalent compliance with the natural vascular tissue, the inconsonant response of the grafted portion results in fluid turbulence and direct tissue damage at the sutured juncture. If the diameter of the fluid path is large or if the rate of fluid flow is high, the adverse effects of thrombosis may not be severe. This is true, for example, in the larger vessels such as the aorta which has both large diameters and substantial blood flow. If, however, these favorable conditions are not present, blood clots accumulate and frequently result in occlusion of the fluid path. For this reason, none of the previous grafts (polymer, Dacron, or ceramic) have been effective in the venous side of the circulatory system or where the vessel diameter has been less than 6 mm on the arterial side. The combination of minimal diameter and/or reduced blood flow have precluded the effective use of synthetic material for such vascular grafts.
Because of the unavailability of suitable synthetic materials with the required compliance characteristics, vascular grafts for small diameter blood vessels now require the transplantation of saphenous vein from the leg of the patient or other vein material from the less critical parts of the circulatory system. This procedure is limited, however, due to potential risks of resultant circulation failure, particularly in older persons. Furthermore, it is not uncommon for an older patient to have failing saphenous veins, requiring the use of potential donors with the accompanying risks of antigenic reactions. The seriousness of these limitations is illustrated by the fact that approximately 60% of the amputations currently performed in hospitals are the result of vascular failure.
Similarly, rejection of prosthetic materials in other body systems has been commonly experienced. Frequently, the treatment of such areas as the common bile duct, urethra, ureta and hydrocephalic tubes includes the need of tissue replacement which has previously been unsuccessful due to the concurrent needs of compatibility and mechanical compliance. Such requirements are not satisfied by synthetic materials now available in the commercial market.